1. Field of the Invention
This invention relates generally to a radio receiver, and more particularly to a novel double superheterodyne receiver having a phase locked loop (PLL).
2. Description of the Prior Art
In general, the prior art radio receiver employs a superheterodyne. As well known, the superheterodyne receiver converts a received modulated carrier signal to a second radio frequency carrier signal whose frequency is lower than that of the received carrier signal. The second radio frequency is called as an intermediate frequency (IF). By converting the carrier of high frequency to that of low frequency, it can be designed to provide much higher and more uniform amplification and selectivity per stage over the entire tuning range of the receiver, and the signal-selection circuits in the IF amplifier are permanently fixed-tuned to the particular frequency employed in it. They can be designed for optimum performance as this frequency. In general, the intermediate frequency (IF) is selected 455 KH.sub.z.
Further, in the art there has been proposed an AM radio receiver or special receiver of double superheterodyne type for receiving broadcasting waves of higher frequency in which an incoming signal is not converted to the intermediate frequency signal of 455 KH.sub.z by a mixer directly but the incoming signal is converted to a signal whose frequency is higher than 455 KH.sub.z but lower than that of the incoming signal at the front stage of the mixer.
Such a type of receiver includes first and second separate oscillators, and first and second mixers, respectively. For example, the incoming signal is converted to a signal with the frequency of 45.145 MH.sub.z (first IF signal) by the local signal generated by the first oscillator, and then the first IF signal is converted to a signal with the frequency of 455 KH.sub.z (second If signal) by the local signal generated from the second oscillator. As described just above, in the prior art double superheterodyne receiver the incoming signal is frequency-converted twice to receive broadcasting waves of high frequency stably.
As set forth above, the double superheterodyne receiver generally includes two separate local oscillators, so that it is required that the two oscillators always produce local signals so as to provide first and second IF signals correctly. Especially, when there is a drift in the frequency of the local signal which is generated by the second oscillator and supplied to the second mixer to produce the second IF signal, it can not be possible to produce the second IF signal having a predetermined frequency, for example, 455 KH.sub.z. If the above frequency drift exists, the detector provided at the rear stage of the IF stage will produce a distorted audio signal.
Recently, such a receiver has been proposed which comprises a tuner, so called PLL (phase locked loop) using a reference oscillator and a voltage controlled oscillator. The tuner using the PLL can be, of course, adapted to a double superheterodyne receiver. It is, however, expensive to use two PLLs. In addition, when the frequency of a received signal is displayed or a display device by using the signal from the PLL, the error operation of a counter oftenly causes an error frequency display and a flicker in display.